FIG. 1 shows a drive circuit such as this in which, by way of example, an LED strand is formed by four LEDs D1 to D4. A switch T1 is arranged at one end of the LED strand which is connected on the one hand to a control loop and on the other hand to a supply voltage UV. At the other end, the LED strand is connected to ground via a shunt resistor RSh. The voltage USh which is dropped across the resistor RSh is supplied to an integrator 10 which produces at its output a variable which corresponds to the mean value īLED of the current iLED flowing through the LED strand. The variable iLEDact corresponding to the actual mean value of the current īLED is supplied to an input of a comparator 12, to whose other input a variable is supplied which corresponds to a nominal value of the current īLED through the LED strand, namely īLEDnom. The comparator 12 provides a control voltage UControl at its output, and this is supplied to a further comparator 14. The triangular waveform voltage UD which is produced by a triangular waveform generator 16 is applied to its second input. Its output is connected to the control input of the switch T1. The mean value īLED of the current through the LED strand can be varied by varying the value īLEDnom, thus varying the brightness of the light which is emitted by the LEDs D1 to D4, that is to say dimming them.
This drive circuit has a number of disadvantages: for example, when an LED strand such as this is operated in a motor vehicle, it must be expected that the supply voltage UV, for example the vehicle power supply system voltage, is not constant. The trimming of the number of LEDs in the LED strand must in this case be chosen so as to make it possible to achieve a sufficiently high current through the LED strand even when the supply voltage UV is at its minimum, in order to ensure a certain minimum brightness of the LEDs. If the total supply voltage is now always applied to the LED strand in order to achieve high efficiency, any increase in the supply voltage leads to an increase in the peak current flowing through the LED strand, in this context see the profiles shown by thin lines in the central illustration in FIG. 2. If, for example, the air-conditioning system in a motor vehicle is now switched on, this sudden voltage change can lead to a sudden change in the supply voltage which is available for supplying the LED strand. In the case of some LEDs, in particular in the case of InGaN-LEDs, a different peak current leads, however, to a shift in the wavelength of the light which is emitted by the LED, which can then be perceived in a disturbing manner.
A further disadvantage results from the fact that the LEDs have a negative temperature coefficient of several millivolts per degree Celsius. In this context, reference should be made to the illustration in FIG. 2, in which thick lines are used to show, by way of example, the peak current îLED for various temperatures. Although the same mean value īLED is always set in all three illustrations, the peak current îLED varies considerably. At low temperatures, see the illustration on the left, the peak current is very much lower than at higher temperatures, see the right-hand illustration. In order to achieve the same mean value īLED, the LED strand is supplied in a pulsed manner, in which case the pauses between two successive pulses must be chosen to be greater at higher temperatures. However, the different peak current in turn results in an undesirable change to the wavelength of the light which is emitted by the LEDs.
Alternatively, in order to reduce the effects of fluctuations in the supply voltage and in the ambient temperature, it is possible to provide for a bias resistor to be connected upstream of the switching element. However, this results in poor efficiency. A further disadvantage is that the energy which is consumed in the bias resistor leads to a further increase in the ambient temperature, and thus exacerbates the negative effect.